Self-regulating, electrically semi-conductive, heating cables are well known in the art. They generally feature at least one pair of elongate electrical conductors such as stranded or solid copper wires that are spaced apart from each coextensively along the length of the cable and are embedded within and electrically interconnected by means of a semi-conductive composition that typically comprises one or more polymeric materials such as a polyolefin or fluorocarbon or chlorofluorocarbon materials containing an amount of uniformly dispersed electrically conductive particles sufficient to impart the degree of semi-conductivity for the current regulating characteristics desired. Examples of self-regulating cables of the type described above are disclosed for example in U.S. Pat. Nos. 3,858,144; 4,188,276; 4,200,973; 4,277,673; 4,327,480; 4,334,351; and 4,334,148, the disclosures of which are incorporated herein by reference.
Typically, the semi-conductive composition is a composition that exhibits a positive temperature coefficient (PTC) characterized by exhibiting increasing electrical resistance with increasing temperatures up to a temperature at which the resistance is high enough to prevent current flow for the particular voltage applied across the spaced apart conductors. In making such cables, it is common practice to enclose the semi-conductive composition with a shape-retaining jacket that has a melt point temperature higher than that of the semi-conductive composition and then anneal the combination at a temperature at or above the melt point of the semi-conductive composition and below the melt point of the jacket material for a period of time necessary to reduce the volume resistivity of the semi-conductive composition to the level desired while preventing shifting of the conductors and then cross-link the combination by suitable means such as by exposing the combination to high energy electron radiation. Such cables often include one or more additional polymeric and/or metallic jackets about the shape-retaining jacket for added mechanical protection and the like.
The semi-conductive polymeric compositions used in the present invention are generally olefinic polymers and copolymers such as low, medium and high density polyethylene and blends thereof and polypropylene polymers and copolymers and blends thereof having at least about 20% crystallinity as determined by x-ray diffraction as is well known in the art. Commonly used for lower operating temperature self-regulating heating cables are copolymers of ethylene (e.g. polyethylene) and vinyl acetate or ethylene-ethyl acetate copolymers. Understandably, the semi-conductive composition may exhibit one or more crystalline melting temperatures depending on the nature of the particular combination of polymers and copolymers used. For example, a blend of a low density polyethylene and ethylene-vinyl acetate having an acetate content of about 18%, typically exhibits two crystalline melting points that are about 20.degree.-22.degree. C. apart. The term "olefinic semi-conductive composition" as hereinafter used means a composition that is primarily olefinic in nature by containing a predominate amount of one or more olefinic polymers or copolymers or blends thereof hereinbefore described exhibiting one or more crystalline melt point temperatures that contain an amount of one or more types of electrically conductive particles, preferably carbon black particles, that can be formed, preferably by melt extrusion, about the electrical conductors and possesses sufficient crystallinity to provide the self-regulating characteristics desired.
The olefinic semi-conductive composition used in the present invention may also include effective amounts of additional ingredients such as anti-oxidants, heat stabilizers, processing aids and the like provided they do not interfere with the processing and self regulating characteristics desired.
Depending upon the degree of semi-conductivity desired, typically from about 5% to about 25% and more commonly from about 10% to about 25% and even more preferably from about 17% to about 22% by weight of one or more electrically conductive particles to the total weight of the composition are uniformly blended with the olefinic polymer or copolymer to provide the olefinic semi-conductive composition which is then formed, preferably by melt extrusion, about the spaced-apart electrical conductors as hereinbefore described. Preferably, the conductive particles comprise one or more types of electrically conductive carbon black particles of which one particularly suitable type of carbon black is sold under the tradename VULCAN XC-72 by Cabot Corporation.
The shape-retaining jacket used in the invention is made from a thermoplastic polyurethane elastomer that is able to be formed, preferably by melt-extrusion, about the olefinic semi-conductive composition. The use of thermoplastic polyurethane elastomers for the shape retaining jacket is well known such as, for example, the use of a polyurethane elastomer sold under the tradename TEXIN 591-A by Mobay Corporation and by Goodrich Chemical Company under the tradename ESTANE 58305 disclosed in U.S. Pat. No. 3,914,363, the disclosure of which is incorporated herein by reference.
Historically, it has been difficult to bond polyurethane elastomers to polyolefinic materials. Heretofor polyurethane elastomers commonly have been formed about olefinic semi-conductive compositions by melt extruding a tube of the elastomer about the semi-conductive composition and then drawing the tube snugly about the composition by applying a vacuum within the tube as disclosed, for example, in U.S. Pat. No. 4,286,376. It has been discovered, however, that under such conditions and without more the polyurethane elastomer exhibits little or no melt fusion bond to the olefinic semi-conductive composition and is subject to imperfections such as sink marks and bubbles and the like as well as having reduced mechanical integrity such as in the form of wrinkling in the region of bending and is subject to underdesirable changes in conductivity due to exposure to moisture that is able to penetrate the semi-conductive composition by migrating along the cable between the semi-conductive composition and the jacket. It is also well known as, for example, disclosed in U.S. Pat. No. 4,334,351, the disclosure of which is incorporated herein by reference, that the handling characteristics of a laminate structure is greatly improved when a substantial bond exists between the layers of the laminate enabling it to act as a unitary structure.
In view of the above, a need exists to provide a means by which to improve the mechanical and electrical integrity of a self-regulating heating cable of the type utilizing an olefinic semi-conductive composition enclosed by a polyurethane elastomer shape-retaining jacket as well as eliminate imperfections heretofore associated with such jackets in the past in addition to improving the overall handling characteristics of the cable.